let SwTextBoxHelper::findTextBoxes() take a const SwDoc

Change-Id: I173b85c15fb59f4fb0bbdf5befdc998aabc27565
SwTextBoxHelper::findTextBox: only draw frames can have textboxes

Change-Id: Id351b96d616eecc780e987a1d0e99ca3f40e4b7c
SwXFrames::getCount: ignore TextFrames, which are TextBoxes

Change-Id: I39fb4fcff2a55cef658d93c80c1cf448bd2e7c86
SwXFrames::getByIndex: ignore textboxes

Change-Id: I0d0f16e147092402b85d84e90fe8ee9812a2937f
SwXFrameEnumeration: ignore textboxes

Change-Id: Ifd8ff27bfb11b1162197e7a159c5bbe25b7d9bc3