Renovate kdevelop bash completion file

Allows to complete the session names in `kdevelop -s ...` on the
command-line.

File will be installed to the standard bash completions directory
(figured out by querying pkg-config during CMake configure time).

Manual usage:

Under Bash:
  source kdevelop_completions.bash

Under ZSH:
  autoload bashcompinit
  bashcompinit
  source kdevelop_completions.bash

  (cf. https://stackoverflow.com/questions/3249432/can-a-bash-tab-completion-script-be-used-in-zsh)

Example output:
```
  % kdevelop -s kde
kde4      kdevelop-production  kdevtest
```