gitk: support link color in the Preferences dialog

gitk: support link color in the Preferences dialog

As a dark-theme user, I use the Preferences dialog to set colors
for gitk. The only color I cannot change via that dialog is the
link foreground color, which leads to using the default link color
on a dark background that makes it hard to read.

Make the link foreground color also configurable in the Gitk
Preferences dialog's Color tab, so users won't need to dig into
the code/manual to check if it is configurable and can simply set
the color there.

Signed-off-by: Wang Zichong <wangzichong@deepin.org>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>