Make mass update using checkboxes more intuitive

[iffy]

Some ideas on how to make this flow more intuitive (you could incorporate one or more as fit):

• If the Apply button is clicked when nothing is selected, show a messagebox like "No items selected".
• Make the Apply button disabled when nothing is selected. This way the user will figure out that something is missing / not ready (with no need for adding a new message text which requires extra translations).
• Make the Apply button show zero when nothing is selected (now it starts showing numbers only if something is selected).