Fix and cleanup logic related to frames

There are currently several problems with handling frames that should be resolved. Some of these problems are already covered by existing issues, but this new issue list all these highly related problems in one place. The whole frame handling logic should be rewritten and all these problems resolved at the same time.

1. Page Should (Not) Contain searches text from frames, but only searches from one level and not from sub frames (#98)
2. Other keywords don't look inside frames at all. This includes Page Should (Not) Contain Element and Current Frame Should (Not) Contain. (#98)
3. Page Should (Not) Contain internally selects the main frame, disabling earlier Select Frame calls.(#201)
4. We should see could we deprecate all Current Frame ... keywords. It would be better that after selecting a frame, all subsequent keywords would operate in the frame. This may actually already be the case, except with Page Should (Not) Contain which looks from the root (see 3. above).
5. If all keywords operate on the selected frame, Page Should is pretty misleading wording. Should we consider renaming these keywords like Page Should Contain -> Text Should Be Present, Page Should Contain Element -> Element Should Be Present?

Beginne damit, die aufgeführten frame-bezogenen Schlüsselwörter nachzuverfolgen und die Issues #98 und #201 zu prüfen, da keine Dateien oder Tests genannt werden. Definiere den Abschluss als konsistentes Verhalten für verschachtelte Frames und ausgewählte Frames über die betroffenen Schlüsselwörter hinweg, und kläre anschließend, ob die Namen der Schlüsselwörter Current Frame und Page Should geändert werden sollten.