feature: enable .Xresources control of Xcursor theme for instantwm and st

[ppenguin]

To configure better HiDPI ergonomy, I have a custom cursor theme, which is both set via lxappearance and in ~/.Xresources
and affects the size and icons via Xcursor.theme and Xcursor.size.

This seems to work pretty much everywhere, except:

• over the root window
• over the menu bar
• over the terminal (st+tmux)

Objective: enable seamless integration of the chosen Xcursor theme also for these.

[ppenguin]

Interestingly after I recently executed lxappearance with my chosen cursor theme, t was shown correctly over the root window and the status/menu bar. (And I expect it should work the same when e.g. forcing GTK styles with home-manager)

This means that after integrating instantOS/st-instantos#4 this issue can be closed.