GeanyLua Fix some compiler warnings

* Replace shadowed "app" variable name with still quite short
  "geany->app" and avoid the variable altogether.
* Avoid re-defining C99 "isblank()" function/macro by renaming to
  "is_blank()". Could add some pre-processor checks and only define
  "isblank" pre-C99 but this behaves same as before I think.
* Avoid storing allocated string in SciTextToFind's const member since
  it needs to be passed to g_free() later. Temp var used to store
  non-const pointer but could instead add a cast in the g_free() call
  but I find this a little cleaner somehow.
Use single <geanyplugin.h> that provides everything needed
Merge pull request #149 from codebrainz/geanylua-fixups

Minor GeanyLua fixups