Use constants for the image cache size and cache trim size
Add an option for setting the size of the tile cache

This makes it possible to either set a smaller cache size,
which might lower the maximum amount of memory consumed
by modRana or to increase the cache size if there is enough
memory a eliminate some time reloading (this can be useful
on systems with high display resolution and adequate amount of memory).

About the cache size:
* for the map in the GTK GUI to work correctly, the cache size needs
  to be bigger than teh amount of tiles present on screen at any one time
* to get the number of tiles on screen, just compute how many 256*256 tiles
  fit on it (with 1x scaling)
* Qt 5 GUI should continue to work even if the cache is disabled but tile
  reloading might be more noticeable
* size of an individual tile depends on the amount of details on the given tile,
  so the same amount of tiles can have different overall size in different places
  and at different zoomlevels