Don't add large dirs to the default TM import set.

Don't add $HOME (or even C:\ on Windows!) to the list of directories to
search when creating a new translation memory.  Scanning the directories
is slow and due to bad UI design, Poedit seems unresponsive.

This is not worth fixing properly, because all this TM import will be
reworked completely.

See #510.
Update copyright years.
Implement find shortcuts.

Use the standard Ctrl+(Shift)+G.  On Windows, also use F3 and Shift+F3
for compatibility with some software.

Fixes #283.
Fix broken detection of bad file charset.

Poedit has code to detect if the PO file cannot be saved in the
requested charset and change it to UTF-8 in that case.  Unfortunately,
this code was broken due to incorrect check of conversion failure.
Fixed now.
More robust bad file charset detection.

Previous code detected incompatible charset value by going through all
translations and checking their conversion.  This required updates when
catalog items changed (e.g.  adding plurals) - and these weren't done,
the code was broken for e.g.  plurals or comments.

Instead, serialize the catalog into text file *first* and verify each
and every line.  This catches conversion problems in all parts of an
item and guaranteed that no parts will be overlooked ever again.
Update version to 1.5.5.