ui: content view: fix use of external static tbar declaration

ui: content view: fix use of external static tbar declaration

when using a declarative `tbar` when instantiating a ContentView,
it is a reference and gets pushed into every time.

instead, copy the given tbar into a local variable and use that

this fixes an issue that we push the same components (e.g. search) into
the toolbar whenever we open a view with a new ContentView
(when using declarative tbar)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>