Provide ability to configure size cut-off for local file previews

Summary:
FileWidgets read from kdeglobals the property "MaximumSize" under "PreviewSettings" to decide if a preview will be generated for that file.
There is no current GUI to change that file size limit. On the other hand Dolphin ignores it.

This patch aims to fix that by adding new configuration options to Dolphin. That is a new spinbox in Dolphin settings under General -> Previews tab.

Test Plan:
1 - Set up a local folder with 2 jpg images of less and more than 1 MB respectively.
2 - Go to Dolphin Preferences. General -> Previews and check "JPEG Images" from
    the list. And set "Skip previews for files above:" to 1MB.
3 - Navigate to the afore mentioned folder. Only the image of size less than 1 MB should
    show a preview.

BUG: 331240

Reviewers: ngraham, #dolphin, meven, elvisangelaccio

Reviewed By: ngraham, #dolphin, meven, elvisangelaccio

Subscribers: cfeck, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D28402