unify all Dave messages to ensure that there are always translated

The root cause is that gettext does not dig into scala and python
files for translated strings
Specify a working font for plain text to help the poor Mac users
welcome.RunHalf: fix the scala templating
welcome.FlowerPot: fix the french translation (growFlowers with a s)
fix a few typos in French translation
FeedbackDialog: reset the text on each invocation
more french typos
welcome.MethodsArgs: Fix a faulty copy/paste in scala prototypes
welcome.LoopFor: Fix the scala syntax of for loops
welcome.RunHalf: the description and the given condition did not match