edited awkward phrasing. changed 'local copies' to 'cache locally'.

At least on my computer (running Fedora Linux and Darktable 1.4.2), the
button in Selected Images panel in lighttable mode is labeled 'cache
locally', not 'local copies', so I changed the reference in the manual
page here to be more consistent.
Merge branch 'master' of https://github.com/darktable-org/darktable into clarifying-text-in-user-manual
further tweaking of prose in localcopies.xml

Following the discussion
[here](http://www.darktable.org/redmine/issues/9955)
I changed my "local cache" edits back to "local copies." The feature
name change should be reflected in the application source too.

I changed some references to "external storage" to merely "main
storage" and the like, because I think the use case for this feature
applies to any sort of storage that would be impractical to carry
around, not just to RAID NAS.
edited prose for readability. added some extra information about feature.

A couple times I changed "multiple images" to "one or more images,"
because the rating and labeling methods for multiple images work just
as well for single images.
fixing a few things I missed.
minor typography update: changed ` - ` to actual emdash (`&mdash;`).
edited for clarity and readability.
Since we depend on glib-2.30, remove obsolete GLIB_CHECK_VERSION calls
exif: legacy panasonic RAWs have basic exif in the makernote

i worked on this for an hour before noticing this was bleeding
obvious.

this really is one of those cases where you really think:
what were they thinking
Merge branch 'master' of https://github.com/darktable-org/darktable
lua : add an empty wrapper for export_dispatched
exif: finish legacy panasonic handling
adobe_coeff: minor fixes for panasonic and leica
rawspeed: Canon PowerShot S120 support
rawspeed: initial legacy panasonic and leica cropping
Merge pull request #565 from Terrycloth/clarifying-text-in-user-manual

Third batch of edits to user manual. (See individual commits for details.)
Revert "Merge pull request #565 from Terrycloth/clarifying-text-in-user-manual"

This reverts commit 8bcd8822f28926a7ac728537a6f92fc105f369ce, reversing
changes made to 8c5f2084e7823b88a45a18ee5a79aaadfbae4642.
usermanual: fix missing &mdash; entity in recent merge of pull request
Revert "Revert "Merge pull request #565 from Terrycloth/clarifying-text-in-user-manual""

This reverts commit 59dc503f25fe5d29fb5b7ce0a88038f0fe0691c7.
usermanual: add missing DOCTYPE preambles to xml files