Refactored Rename Tab dialog and its context menu (#2713)

* Added placeholder to tab rename dialog's input

* Always set placeholder to default tab name.

* Renamed context menu entries, updated popup

* Removed TextInputDialog class, slight popup fix

* Forgot to rename variable (no fun allowed 😥)

* forsenT

* Made use of QDialogButtonBox

* Added changelog entry

chatterino.pro

@@ -276,7 +276,6 @@ SOURCES += \
     src/widgets/dialogs/switcher/NewTabItem.cpp \
     src/widgets/dialogs/switcher/QuickSwitcherPopup.cpp \
     src/widgets/dialogs/switcher/SwitchSplitItem.cpp \
-    src/widgets/dialogs/TextInputDialog.cpp \
     src/widgets/dialogs/UpdateDialog.cpp \
     src/widgets/dialogs/UserInfoPopup.cpp \
     src/widgets/dialogs/WelcomeDialog.cpp \
@@ -533,7 +532,6 @@ HEADERS += \
     src/widgets/dialogs/switcher/QuickSwitcherModel.hpp \
     src/widgets/dialogs/switcher/QuickSwitcherPopup.hpp \
     src/widgets/dialogs/switcher/SwitchSplitItem.hpp \
-    src/widgets/dialogs/TextInputDialog.hpp \
     src/widgets/dialogs/UpdateDialog.hpp \
     src/widgets/dialogs/UserInfoPopup.hpp \
     src/widgets/dialogs/WelcomeDialog.hpp \