Make MODKEY flexible

[mrwsl]

If one changes

dwm-flexipatch/config.def.h

Line 714 in 96820b2

#define MODKEY Mod1Mask

for example to Mod4Mask a lot of the shortcuts become ambiguous.

So the idea would be some sort of case:
Based on the value of the MODKEY define an ALTMODKEY that has either the value Mod4Key or Mod1Key.

[bakkeby]

Not sure if that makes it more intuitive.

Perhaps it is better to have a more generic search and replace along these lines:

sed -i 's/Mod4Mask/Mod1Mask/;s/#define MODKEY Mod1Mask/#define MODKEY Mod4Mask/' config.h

[mrwsl]

I actually did it like this and thought my idea would improve the use a different modkey. But it is just an idea :)