Patchset 2021.4

This merge request contains 4 patches:

1. Remove unused options.dialog files

2. Surround declarations with G_BEGIN_DECLS and G_END_DECLS

3. Move text inside the tachometer:

   This patch slightly improves readability of tachometer texts by putting the baseline of the text 1 pixel above the center of the tachometer circle.

   Before:

   After:

   Closes: #20 (closed)

4. Enable automatic switching of colors based on light or dark UI theme:

   The colors in the configuration dialog can now be set to empty strings, in which case the color of the text or of the bar will be set according to the current UI style.

   Remaining issues to be fixed:

   • There seems to be an interference between Gtk CSS providers applied to a bar if its color is an empty string
   • There is currently no help text nor a tooltip telling the user that the color can be set to an empty string