Fixed: moved undo/redo functionality to EditPane.js

Also clarified formatting of original "append" statement.
New: added help drop down menu

-Added brief help menus for default plugins.
-Added documentation in PluginTemplate.
New: created utils.js for non-vital functions

-Moved functions from meiEditor.js
-Changed plugin files to use global functions instead of meiEditor.function()
-Updated documentation in PluginTemplate accordingly.
Fixed: two bugs with file uploading

-File inputs are now reset when the modal is reloaded (deleted on close)

-Previously, changing a file input would create a new file input object regardless of whether it was needed.

Consolidated a confusing event listener series into one function called in multiple places now.

Resolves part of my to-do list.
New: find and replace options for "Edit" dropdown

Used built-in Ace find/replace dropdown boxes, only one appears per editor and automatically disappears when tab is switched. Relatively easy.

Also finished taking template code out of EditPane. Oops.
Fixed: a few possible bugs with file renaming

-Fixed bug where old filenames were not getting deleted from settings.pageData when renamed.

-Forbid filenames with names that had no alphanumeric characters (would result in a blank JavaScript ID)

-Forbid filenames that duplicated other filenames when both were stripped of alphanumeric characters (to avoid overlap with JavaScript IDs).

-In solving this, Tim told me he didn't realize an error message popped up on the console. I changed the border highlighting to whole-div highlighting to help emphasize it, and added warn/error/log colours.

This introduced a new bug I'll fix in the next commit.