Implement setf & setb

[mjsir911]

Hi,

Currently, it seems like tcell only supports setting the colors using setfg & setbg.

This has left a hole in functionality, resulting in many issues in projects using it:

• Set transparent background for TreeView rivo/tview#314
• Theming: Get Default Color Values from Terminal Bios-Marcel/cordless#157
• Feature Request: Dark Theme gcla/termshark#17
• The default theme has changed (doesn't use the terminals color set anymore?) rivo/tview#252
• Feature request: allow user to customize background color. browsh-org/browsh#279
• Support setting background color wtfutil/wtf#84
• Get existing background colour of terminal #292

Supporting setf & setb will allow using colors with custom colorsets, which respect the terminal emulator's own theming styles. This saves projects from having to implement their own colorscheme configurations as seen in some of the issues above. This will only be the case when a small amount of colors are being used, but at least some projects do this

Supporting custom semantic colorschemes also help with user's with vision problems who have customized the terminal colorscheme to help with that.

See colortools for how it works on xterm compatible terminals.

default background & foreground

Additionally, on xterm, 39 and 49 are the background and foreground colors respectively. This will help with some of the issues of incorrect background colors. This is both implemented in ncurses & bsd curses, more info found here

[gdamore]

I think there are some easy options here -- we'll need to add some new constant values for "ThemeColors" to distinguish them from explicit requests for colors by RGB. This is better than just using a global switch because it can allow an app to honor theme settings for the terminal for most of the window, but require explicit colors in other places. (E.g. a browser app could use the terminal defaults for most things, but use explicit RGB values for images.)

[antonmedv]

It will be cool to have bold working too.