Change string concatenation and plurals to facilitate localization

[Vojtěch Perník]

Strings of the type:

• %d Pages
• %d of %d Pages
• %d Entries
• %d Total Results

and similar should rather be written using the ngettext method. This removes the hassle of figuring out whether to use singular or plural.

Strings of the type:

• Unable to Import Library
• Fields
• Constraints

and similar strings containing a space should be constructed using wildcards like %s (Unable to Import Library %s). Many languages have a different word order than English. In addition, this last two strings should be written again using ngettext.

The error message in the src/Partials/TreeBuilder.vala file on line 132 should be constructed in the same way.

Thank you in advance.