Build: More improvements to the build system

 * This patch is another one in a series of improvments
   to the build system.
 * It fixes some 'make distcheck' problems that the
   patch to compile the UI file into the tilda binary
   introduced.
 * It also removes some unneeded includes.
 * It switches to using PKG_CHECK_MODULES for X11 detection
 * Finally it makes out-of-tree builds possible. To perform an
   out-of-tree build do the following:

    - create a such as build/ either inside or outside the tilda folder.
    - run /path/to/tilda/autogen.sh && make
    - optionally run 'make distcheck'

   This will help you keep your source directory clean.

 * Use ""-includes instead of <>-includes for local headers
Use strncasecmp instead of array lookups

 * Thanks to Ghoughpteighbteau <lierorules@gmail.com>
Update po/ directory
Make getPids() static
Option to select how pull-up behave on non-focused terminal

- Focus Terminal: put focus back on the terminal window instead of hiding it.
- Hide Terminal: hide terminal window instead of putting the focus back to it.