refactor MasterLayoutDialog::remove to SdPage::DestroyDefaultPresObj

Change-Id: If4edb16b2e94396249f3e2ba99381fcd8341d790
refactor delete code into FuDeleteSelectedObjects

Change-Id: If6929dfae5e284b1201da5505b9c4e5abceedc6d
implement toggling off removeable master elements with direct delete

i.e. selecting footers/headers/slidenumber/datetime previes placeholders in
master page view and pressing delete will toggle those master elements off.

Change-Id: I91f745703e43cbb4fdd037da4eab7c8f6bf9fbf5