Also update to the tcell v2 PaletteColor api, which should keep the chosen
theme of the user intact.

Note, that if $TRUECOLOR is defined and a truecolor given, aerc will now stop
clipping the value to one of the theme colors.
Generally this is desired behaviour though.

Introduce the ability to configure stylesets, allowing customization of
aerc's look (color scheme, font weight, etc). Default styleset is
installed to /path/to/aerc/stylesets/default.

This reverts commit 1ff687ca2b0821c2cacc1fa725abb3302d2af9da.

Introduce the ability to configure stylesets, allowing customization of
aerc's look (color scheme, font weight, etc). Default styleset is
installed to /path/to/aerc/stylesets/default.

The grid used static sizes which meant that changing settings didn't
have an effect on elements of the ui, notably the sidebar width. This
patch makes the `Size` parameter of a cell a function which returns the
`int`, allowing for dynamic sizes.

A `Const` function is also included for ease of use for static sizes.

This reverts commit 0f78f06610c0e8887aba2ae50e99b86477a384b3.

The following functionalities are added to configure aerc ui styles.
- Read stylesets from file with very basic fnmatch wildcard matching
- Add default styleset
- Support different stylesets as part of UiConfig allowing contextual
  styles.
- Move widgets/ui elements to use the stylesets.
- Add configuration manual for the styleset