feat: Remove clipboard items older than set time

[Aksellercan]

This PR adds a function to remove items older than set time in settings under Storage options. Function is called when program is loaded or a new item is added to it. This function wont clear pinned items and to avoid unnecessary cycles function will exit early if clipboard is empty or user sets time to 0.

Function can also run in the background without being triggered by add or load using "checkForChangesInPasteboard" function. However, this will run it twice a second from my testing so, I added "lastChange" variable to store last time it cleared the old items. I chose half of set time in hours as condition if it's larger then it older items will be cleared and sets "lastChange" to now.

Time setting default is 8 and accepts values from 0 to 48, setting it to 0 will disable it.

Added:
Background cleanup
Cleanup interval (Time set / 2) to avoid unnecessary cpu cycles
Settings entry.
removeOldestItem() function