Add priority/sorting for AgentTicketZoom MenuModules

[itweserems]

In the current (and probably all previous) Znuny version, the MenuModule entries for AgentTicketZoom are sorted by key-numbers of their SysConfig entries, e.g. "000-Back", "210-Print".
In current dev code-branch this sorting happens in Kernel/Modules/AgentTicketZoom.pm, starting on:

Znuny/Kernel/Modules/AgentTicketZoom.pm

Line 1233 in f28d711

for my $Item ( sort keys %ZoomMenuItems ) {

This should instead be done by a priority config, as it is done for other menus within Znuny.

For example, admins should be able to change a "priority" value, e.g. for "Ticket::Frontend::MenuModule###000-Back" that defines it's menu position, compared to the priorities of other MenuModule items.

I am currently preparing an additional option per MenuModule where sorting of MenuModule items in AgentTicketZoom is not based on the SysConfig-key but a new "Priority" parameter instead.

[rkaldung]

@itweserems I labeled the issue/MR as verified to have it merged. It is be possible that one of the developers requests some changes.