Separating ttkthemes and tkimg

[RedFantom]

As described in #22, separating tkimg from ttkthemes would improve platform compatibility, possibly negate issues with loading the library and still provide the possibility for all platforms to load all themes.

My design proposal is as follows:
Build a separate package named tkimg, that provides a function to load the tkimg library into a given Tcl interpreter (belonging to a Tk instance). This package could be installed with an installation parameter.
The themes can be split into two categories: plain GIF-themes, that always can be loaded by Tkinter, and PNG-themes. Currently, only theme arc is PNG-based. Its images should be converted to GIF-format (d2b75c9), then the artifacts should be fixed. Then the following folder structure could be used for the themes:

themes/
├ basic
|  ├ plastik
|  ├ kroc
|  └ basic.tcl (pkgIndex.tcl eq.)
├ gif
|  ├ arc (with GIF-files)
|  └ gif.tcl (pkgIndex.tcl eq.)
└ png
   ├ arc (with PNG-files)
   └ png.tcl (pkgIndex.tcl eq.)

Then, based on whether TkImg could be loaded or Tk 8.6 is present, either png.tcl or gif.tcl is evaluated.

Thus, in steps:

• Build TkImg package and publish to PyPI
• Fix conversion artifacts in arc theme
• Build folder structure and write different package index files
• Remove TkImg from ttkthemes and implement installation option
• Implement TkImg check for set_theme_advanced

[RedFantom]

This issue is progressing over here.

[RedFantom]

Done and done!