Fix typos and capitalization

[akhial]

Fixed typos and capitalization :heavy_check_mark:

Used the GNOME interface writing guidelines found here: https://developer.gnome.org/hig/stable/writing-style.html.en

Changes:

• Added "..." after menu items that require further interaction

• Change "UserName" and "PassWord" to "Username" and "Password" respectively

• Apply "title case" to title style headings and "sentence case" to conversational headings E.g., "Number Of Connections" becomes "Number of Connections" when not before checkbox "Download options" becomes "Download Options"

• Use "title case" for buttons, menus, etc

• Use "sentence case" for labels which lead to user input such as text fields and checkboxes

• Change ", etc..." to ", etc.". Only one of ", etc." and "..." should be used because they are functionally identical

• Change "Muxing" to "Mixing"

• Remove periods (.) after checkbox labels in preferences UI

• Other various typos ^^

Thanks for the great work! :slightly_smiling_face:

[alireza-amirsamimi]

Thanks for your pull request. I merged this pull request successfully. I have no idea about this conflict error :)