Pyslvs-ui

**Features**:

+ Correction of link length range in dimensional synthesis page.
+ Change the behavior of adding points on main canvas.
+ Add duplicate function for graph collections page.
+ Fix the error of switching structure collections.
+ Showing attributes instead doing number synthesis after generalization.
+ Documentation update:
+ APIs of kernels.
+ Two major IO formats.
+ Change the format of signatures.
+ Change the theme to "material".
+ Adjust "zoom to fit" action during dimensional synthesis page.

**Development**:

+ Improve Makefile build and clean command of submodules.
+ Add backend data instead of UI table items.
+ Attributes refactor of file system.
+ Submodule import from namespace directly, instead of `core.libs` module.

**Features**:

+ Change the list widget of graph atlas into tree widget.
+ Add step progress bar at synthesis page.
+ UI improvement of synthesis pages.
+ Add slides links in documentation.

**Development**:

+ Disable warning about YAML loader.
+ Avoid PyQt crashes on exit.

**Features**:

+ Interface adjustment of menu bar.
+ Correction of loop chains.
+ Capture image of main canvas before choosing path instead of after chosen.
+ Documentation updated:
+ Main functions.
+ API documentation of Python-Solvespace.
+ Check cut-links for collections.
+ Add "Six bar linkage mechanism" configuration.
+ Add progress dialog on contracted link assortment enumeration.
+ Nested do loop method for contracted graph enumeration.
+ Speed up performance of searching contracted graphs.
+ Add clear assortment button.
+ Optimized user experience of free move mode.
+ Allow empty links in mechanism expression grammar.
+ Fix the crash of restoring a mechanism expression.

**Development**:

+ Save user settings to "~/.pyslvs.ini", let it can be backup more easily.
+ Speed up performance of number synthesis.
+ Speed up performance of Cartesian product function.
+ Switch synthesis functions into QThreads.
+ Add libraries doc for development.
+ Type of `__version__` is changed to string.
+ Core kernels `pyslvs`, `python-solvespace` have been changed to PyPA package.

**Features**:

+ Add basic Readthedocs and MkDocs project for user manual.

**Development**:

+ Contracted graph enumeration algorithm:
+ Speed up performance for searching between link assortment.

**Features**:

+ Show a progress dialog when loading file or database.
+ CAD kernels improvements.
+ Performance improvement of algorithms.
+ Monochrome mode for mechanism. (Excluding indicators)

**Development**:

+ Remove related import at "core" (root module) to speed up the compile time.
+ Remove customized splash class that is used once.
+ Update new Cython wrapper of Python-Solvespace.
+ Removed SWIG requirement.
+ Removed the support of Eric IDE.
+ Basic logging method instead of "print" function.
+ Now Pyslvs will create a log file beside executable.
+ The log file will be delete after generally closed Pyslvs.
+ Add startup time record.
+ Test the kernels when packing executable.
+ Performance improvement if dimensional synthesis algorithm.
+ Add code of conduct.

**Features**:

+ Add single graph saving function for synthesis collection panel.
+ UI description improvements.
+ Removed "ground enumerate" button of synthesis collection panel.
+ Fix the bug of deleting a point is inside the input pairs.
+ Fix the bug of limitation option when algorithm option loaded.
+ Widget focus behavior correction of tables.
+ Fix the bug of saving a YAML project.
+ Remove "solution" preview function from preview canvas class.
+ Adjustment margin size of preview window.
+ Support the "Categories" key of AppImage desktop file.
+ Enhancement of "link adjust" function.
+ Enhancement of mechanism grammar.
+ Set the table headers to auto resize mode.
+ Change the merge behavior of synthesis result as add to storage.
+ New style selection tip widget instead of QToolTip.
+ Solve the memory leak of Qt dialogs.
+ Unified the IO encoding of text-based files.

**Development**:

+ Stop support Python 3.6 or below.