Fixes for sequences

Fixes #1054 (closed):

• adds a normalize_seqname function and use it on arg->destroot in convert_thread_worker so that the final seqname and the name that is stored in destroot are consistent. This enables to load the sequence at the end of conversion when there is a . in it

and #962 (closed):

• complement list of forbidden characters in filenames
• make allowed output sequence names behavior consistent between GUI and commands (merge, convertraw, convert and link)

Not in a ticket:

• avoid writing conversion report if no file got converted (see d36cb281)