Fix font duplication

[mihaitodor]

This is an attempt to partially address #865 and clean up some inconsistencies in the font-related verbs.

• Add verb w_try_cp_font_files, which makes sure that we don't get the same font file but with different casing under C:\windows\Fonts. It also forces lower case for font names to help alleviate duplication issues.
• Cache PowerPointViewer.exe in "$W_CACHE/PowerPointViewer" for consistency.
• Remove "# FIXME: why is this commented out? Should be removed or enabled." comment. This was referring to the Andalé Mono font, which is no longer distributed with Windows. If you wish, I can create a separate verb for it since it's available here, but I think we don't need it at all, since it's almost identical to Courier.
• Remove chmod +w "${W_FONTSDIR_UNIX}"/tahoma*.ttf since no app should be allowed to overwrite fonts. I'd rather have this posted as a workaround on https://appdb.winehq.org/ instead of maintaining it in Winetricks.
• Place font package URLs in quotes and leave them hardcoded in the verb body because they're much easier to grep for and replace that way.
• w_try_cabextract already passes -q to cabextract.

Note: Since eufonts only updates trebucbd.ttf, allfonts will skip installing eufonts after this PR, since trebucbd.ttf is installed in advance by corefonts. #885 needs to be addressed in order to fix this.

Note2: eufonts is an update to some of the fonts installed by corefonts, so corefonts should always be installed first. Luckily, allfonts installs them in the right order.

As a reference, here is the difference between installed font files based on this PR:
original_font_files.txt
updated_font_files.txt

And here is the difference in registry entries:
original_windows_nt.txt
updated_windows_nt.txt

The observed differences in the registry are due to the fact that eufonts was not installed as explained above.

As described in CONTRIBUTING.md (#871), I ran ./tests/shell-checks successfully and I skipped winetricks-test, since it didn't work as described. See here.

[mihaitodor]

Looks like something went wrong on Travis for the OSX build:

/Users/travis/.travis/job_stages: line 57: pip: command not found

The Linux build was successful.

LE: pip got removed in the latest OSX Travis image: travis-ci/travis-ci#8829

LE2: found a potential fix here: https://github.com/google/nxt-standalone/pull/144/files

LE3: drat, looks like I need to sudo when invoking pip now...

LE4: I opened #890 to tackle this issue first. Once that gets merged, I will rebase this PR.

LE5: I rebased the PR.