tex-beamer{,-doc}: update to 3.70

### Added
- making the aspect ratio available to the user via `\insertaspectratio`
- adding `title`, `author`, `institute`, `date` and `titlegraphic` templates to
  make title page easier to customise

### Changed
- smoothbars outer theme: moved the shading between headline and frametitle a
  bit down to avoid the miniframe appearing clipped off in some pdf viewers
- made top shading of Singapore theme transparent
- scale height of infolines headline with fontsize of `section in head/foot`
- calculate the head/footheight at the start of each frame instead of only at
  the start of the document
- making `\part` compatible with `\nameref`

### Fixed
- adding strut to the `frame number` and `page number` footlines to avoid
  jumping
- fixed bug in `inmargin` inner theme which would show the author indicator
  even if the author was empty

(markd)