Allow mouse selection override using ShiftMask

Similar to xterm or urxvt holding shift before selecting text with the mouse
allows to override copying text. For example in tmux with "mode-mouse on" or
vim (compiled with --with-x), mc, htop, etc.

forceselmod in config.h sets the modifier to use this mode, by default
ShiftMask.

Signed-off-by: Hiltjo Posthuma <hiltjo@codemadness.org>
Merge remote-tracking branch 'suckless/master' into workspace
update config.h
Fix colour-model and simplify xloadcols()

Signed-off-by: Christoph Lohmann <20h@r-36.net>
Fixing trailing whitespaces.
Brightening is again on the TODO list.
Add FRIGN to License, remove trailing whitespace

Signed-off-by: Christoph Lohmann <20h@r-36.net>
Fix some more LICENSE changes.
Merge branch 'master' into workspace

Conflicts:
	st.c