The commit adds some new functions:
* getThumbnailDir()
Returns the path to the thumbnail directory which is a sub-directory of the Cache directory.
There is no need to always create a unique directory after each restart because it doesn't harm
if the thumbnail directoy contains deprecated files.
* createThumbnailDir()
Creates the thumbnail directoy if it doesn't exist yet.
* getSha1Hash
Helper function to compute a SHA-1 hash of a given path. If the same path is passed
then the hash value will be the same.
This way it can be avoided to create a different image file from a project file
after each restart.
* getUniquePNG
Computes the path of a PNG image file for a given project file. It's also possible
to pass an arbitrary string as argument.
* useCachedPNG
If the PNG image exists and if it's newer than the project file True is returned
and False otherwise.
The changes in detail are:
1. For a given project file it is checked if the thumbnail directory already contains
a cached image. If it's newer than the project file it will used, otherwise it will
be re-created.
2. For the system icon to a given file format it is only checked if the icon already exists.
If yes, it will be used.
2bd89e5771