Add option to completely disable directory size counting

Add option to completely disable directory size counting

Dolphin shows the size of directories by listing their contents, which
for some users might cause unwanted load on the file system.
Depending on the size of the subdirectories in question and how the
storage is accessed, this might cause noticeable delays and even
freezing.

This commit adds a new option under "View -> Content Display" that
enables users to set "Folder size:" to "No size", completely disabling
directory size counting. Directory size counting is still enabled by
default.

As a third option for "Folder size" is added, the DirectorySizeCount
boolean setting is replaced with a DirectorySizeMode enum setting. The
old setting is migrated using a kconf_update script.

FEATURE: 477187
GUI: