Cursor: make it possible to change foreground color (#467)

[Vulcalien]

There were some changes to the GUI and to the config file.
Let's talk about the last one first.

Config

Currently, there are only two settings for the cursor colors: cursor_color and cursor_color_fg. Their names can easily confuse, because cursor_color is the background color, while cursor_color_fg is a boolean (False = use a custom background; True = use the foreground as the background).
I removed them both, and replaced them with three new settings: cursor_fg_color, cursor_bg_color and cursor_color_default. The first two are color strings, foreground and background. The last is a boolean (False = custom colors, True = default colors).

Python code

Most of the changes are made to make the new GUI work.
I removed the old signal functions and the old logic that initializes the cursor colors in the preferences menu, replacing everything with the new code.

GUI

I changed the GUI related to the cursor, in the Profiles > Global tab.

I also fixed some inconsistencies that I noticed in the whole menu:

• "Text color" and "Font color" are now "Foreground"
• On the Profiles > Titlebar the Font selector was missing the "indentation"
• "Background color" in Profiles > Colors is now just "Background"

[mattrose]

Working on another feature right now, but I'll take a look at this by monday hopefully

[mattrose]

Wow, this was a lot more work than I thought it was. Great job, and thanks again.