System-monitoring-center

* Code updates for OOP (menus, Performance, Settings)
* Improved performance for tab switches
* Improved FPS counter accuracy
* Fix: Resetting some fields for switched off tabs
* Fix: Showing error messages of startup item commands
* Fix: Closing color chooser dialogs on some systems
* Fix: Speed data conversions for bits/sec and orders
* Minor improvements

* New: Czech language translation
* New: Polish language translation (initial)
* Fix: application instance controls
* Removed: python3-opengl dependency
* Minor improvements

* New: Brazilian Portuguese (pt_BR) translation
* Removed deleted translation lines
* Minor improvements

* Rewrite Startup tab code (improved reliability)
* Add: dependency (python3-gi-cairo)
* Minor improvements for process names > 15 characters
* Several minor improvements

* New: remember window size option (Settings)
* Simplifications for the GUI (several tabs)
* Improved: saving/reading configuration file
* Removed code for reading/writing read-only settings
* Fix: resetting CPU usage graph type (avg./per core)
* Fix: Enabling/Disabling service (Services tab)
* Minor impovements and bug fixes

* Swap Details window is reloaded automatically
* Fix: Showing menu/details window for some processes
* Fix: wrong default_main_tab values
* Fix: disk vendor-model information (Disk Details)
* Update some if controls of main_tab and sub_tab for optimizing
* Minor changes for OS name and version information
* Minor improvements (GUI, translations, etc.)
* Optimize screenshots using png optimization tools