Fix/MultiFile pattern (Issue #696)

[Pr0metheus2]

This is a fix for #696

• Fixed/MultiFile rollover patterns with an index before the extension:

  • RolloverFilenameBuilder: the parser was dropping literal text after the final placeholder, including .log.
  • *$J(.).log now correctly expands: app.log → app.1.log → app.2.log.

• Updated MultiFile mask dialog legend:

  • Fixed incorrectly displayed <date> and <prefix> placeholders in English and German resources.
  • Added both usage examples:
    • *$J(.) → app.log, app.log.1, app.log.2
    • *$J(.).log → app.log, app.1.log, app.2.log
  • Added the examples in English, German, and Chinese.
  • Enlarged the dialog and syntax-help label so the date-format lines are no longer clipped.

Before

After

• Fixed/Items in menu File/Multifile -> "Enable MultiFile" and "File name mask...", were always greyed out. Now they are displayed normally after the .log file is open.
  • removed the BackColor and ForeColor assignments for both:
    • multiFileEnabledStripMenuItem
    • multifileMaskToolStripMenuItem

Before

After

• Added regression coverage:
  • Filename-builder tests for app.1.log and app.2.log.
  • Reader-level test that loads app.log, app.1.log, and app.2.log in correct order using *$J(.).log.
  • Kept the existing *$J(.) coverage for app.log.1 / app.log.2.

Verification:
- Full test suite: 1064 passed, 0 failed, 7 skipped.
- Resource project build: passed with 0 warnings, 0 errors.