Fix segfault on regex deinitialization

[nvlled]

This fixes a segfault error when xdotool is given a broken regex string:

$ xdotool search '*'
Defaulting to search window name, class, classname, and role
Failed to compile regex (return code 13): '*'
Segmentation fault (core dumped)

Some changes are also made so that regexes are compiled only once during initialization. This reduces some overhead when searching through each window entry.