Use a common format for all include guards, make sure each header file
has one and make sure it's named after the file name.
And as a random extra in this commit: Make sure the gen_version.h file
generated by cmake ends with a line break.
Closes#59.
Make it possible to drop database objects (i.e. SQL code) on the tree
view in the Database Structure tab. This is far from being perfect as of
now but enables you to drag and drop entire objects between databases.
Use a custom model for the tree view in the "Database Structure" tab in
the main window, i.e. change from a QTreeWidget to a QTreeView and do
all the item management stuff manually. This might add some code and
complexity but also offers some more flexibility for us.
