Concur

* Add new scatter plot markers: `"|"`, and `"-"`
* Add drag-and-drop functionality (`drag_drop_source`, `drag_drop_target`)
* Add `TF.hover` attribute for more robust hover functionality than `Image.hover_tag`.
* Re-introduce the `maximizied` window flag, because just setting window size to a large number doesn't properly work on Windows
* Fix a [crash on slider and drag widgets](https://github.com/potocpav/python-concur/issues/24)

**Breaking Changes**

Remove the `name` argument from `c.child`

* `concur.core.replace`, `concur.core.replace_tag`, `concur.core.replace_value` added. These make it convenient to transform events without lambdas. Lambdas don't work properly inside loops/comprehensions in Python.

* `concur.widgets.dummy` implemented
* `concur.widgets.input_text` flags argument added. This enables creating a password input field, among other use-cases.
* Make the `imageio` dependency non-mandatory.

* Update ImGui to the newest version in the `docking` branch
* Improve some docstrings

Mainly interactive invocation and documentation improvements.

**Breaking Changes**

* `c.main` argument order switched up to [this](https://potocpav.github.io/python-concur-docs/master/integrations/glfw.html). Widget is now first, then the window name (newly optional), then width and height (also newly optional). This simplifies the Hello, world application and other simple examples considerably.

**Fixes**

* Destroy contexts on `main` exit correctly. This enables multiple `main` invocations with fresh ImGui state.
* Better cleanup on exceptions. Sigkill in IPython doesn't hang the window.

**Breaking Changes**

* `concur.extra_widgets.frame` has new arguments `width`, `height` consistent with `concur.extra_widgets.image`.
* PanZoom-derived widgets now handle hover, down, and drag events in a different way. Instead of being yielded, they are
passed to the `content_gen` as a widget. This makes it possible to react to the events in `content_gen`, which is better for
modularization, and it is newly possible to use the information in `tf` as a part of the reaction. The cost is more complicated
`content_gen` signature. Better solution might be possible. Affected widgets:
* `concur.extra_widgets.pan_zoom`
* `concur.extra_widgets.image`
* `concur.extra_widgets.frame`
* PanZoom-derived widgets are now passed a keyword argument `tf`, instead of a positional argument. This will break code which used other names for the transformation info. Affected widgets:
* `concur.extra_widgets.pan_zoom`
* `concur.extra_widgets.image`
* `concur.extra_widgets.frame`
* `concur.draw.image` argument list was reworked. It now takes additional mandatory arguments `x` and `y`, and non-mandotory arguments `uv_a`, `uv_b`.

**Additions**

* `concur.widgets.font`: render a widget with a specific font

**Fixes**

* `concur.extra_widgets.image` now correctly handles images with dimensions not divisible by 4. Previously, those may get severely distorted.
* `concur.extra_widgets.image` now has transparent background even for images without alpha channel.
* `concur.widgets.key_press`: fix a a crash on glfw.KEY_SPACE keypress.
* Make the system clipboard work by upgrading PyImGui to v1.3.1.
* Add a call to `refresh_font_texture` to GLFW initialization. This enables user to [specify a font](https://pyimgui.readthedocs.io/en/latest/guide/using-fonts.html) and have it integrated, and is hopefully harmless in all other cases.
* Fix a crash on certain greyscale NPOT images
* Fix UI ghosts appearing because the buffer was not cleared.